This invention relates to method and apparatus for guiding an underwater conductor such as drill pipe to a target on a sea floor.
Deep underwater drilling is carried out for a variety of purposes including drilling for oil and natural gas or simply to obtain floor samples from the ocean basin for research purposes. One of the primary problems in underwater drilling is the finding and re-entry of an established hole or ocean floor base to install therein various drilling equipment, to resume drilling following replacement of a worn drill bit, etc. For deep underwater drilling (several miles), this problem of re-entering an open hole or floor base has been likened to standing on a roof top and threading a needle on the ground by moving the upper end of a dangling thread. Because of the difficulty of re-entry, it has oftentimes been necessary simply to abandon a hole before completing the project.
One of the first methods adopted to facilitate hole re-entry involved the use of guide cables extending between the drilling vessel and a guide base positioned over the hole. The drill pipe would be attached to the guide cables and lowered to the hole. This method however is unsuitable for deep underwater drilling simply because of the excessive cost for long guide cables and because the guide cables and drill pipe tend to get tangled as a result of ocean currents and vessel drift.
Another method of underwater hole re-entry involves the use of underwater sonar apparatus to locate the hole and guide the end of the drill pipe to the hole. This method, developed by Edo Western Corp., provides for inserting a sonar transducer assembly inside the drill pipe until the assembly protrudes out the bottom of the pipe. A funnel having a plurality of sonar reflectors spaced about the funnel is positioned over the hole. When the transducer is activated to detect the sonar reflectors, a combination of vessel movement and drill pipe movement effected by a hydrojet at the end of the drill pipe are employed to guide the end of the drill pipe to a position over the funnel. The drill pipe can then be lowered into the funnel and thus into the hole. See Ocean Industry, August, 1970 and Ocean Industry, June, 1970. Although this method of hole re-entry is superior to the guide cable method, it is difficult with this method to re-enter any underwater holes except those equipped with a fairly large funnel since precise location of the drill pipe with respect to the hole cannot be determined.
Another arrangement suggested for underwater hole re-entry is disclosed in R. A. Malott et al., U.S. Pat. No. 3,199,613. With this arrangement, a television camera is lowered through the bore of a drill pipe to the lower end thereof to transmit to a television receiver on board a drilling vessel a view of the sea floor. The lower end of the drill pipe is moved until the hole comes into camera view and then the drill pipe is guided into registry with the hole. This arrangement facilitates precise location of the drill pipe with respect to the hole once the hole is within view of the television camera, but no provision is made for guiding the drill pipe to the vicinity of the hole. Rather, the drill pipe is simply moved in arcs in the general area of the hole until the hole comes into view.
Still another arrangement which seeks to combine the use of sonar and television for underwater hole re-entry is discussed in the publication Off Shore, November, 1970. This arrangement contemplates attaching to the end of a drill pipe a re-entry tool equipped with an underwater television camera and sonar. The sonar is used to guide the drill pipe to the vicinity of the hole and, when the drill pipe is within range for television monitoring, the television camera is activated to provide a view of the sea floor and hole to enable guiding the drill pipe to equipment placed over the hole. Although this arrangement combines the use of a sonar system and television camera, the re-entry package is very bulky and must be attached to the exterior of the drill pipe in which position it may interfere with the coupling of the drill pipe to guide base structure located over the hole. Also, because the television camera is attached to the outside of the drill pipe, it must be tilted at an angle to provide a view of the area directly under the drill pipe and this angular view makes it difficult to guide the drill pipe to the hole. Further, it appears from the aforecited article that the re-entry tool, together with sonar and television camera, remain attached to the drill pipe underwater while the drilling operation is carried out. This not only ties up the sonar and television camera but also subjects them to the debilitating effects of drilling vibration and of being underwater for long periods of time.